1. Field of the Invention
The present invention relates to a multi-branched optical line testing apparatus for determining a faulty line in multi-branched optical lines and for determining the distance to the fault point on the faulty line.
2. Description of the Prior Art
Data, voice data, image data and the like which are handled by a computer are now digitized and communicated via various communication media such as cable and the like.
Taking an example of computer communication for example, only the transfer of data having a relatively small quantity of information, such as character information, compressed static images or the like has been conventionally performed, due to the processing capacity of computers, the low throughput of transfer media and the like.
Recently, the processing speed of computers is rapidly increasing, and there has been a demand for transferring motion picture data using computers. The data quantity in a motion picture becomes larger than that in character information, static images or the like. Furthermore, in order to display a motion picture smoothly on the side receiving the motion picture data, there is a restriction on the time that it is necessary to receive the data constituting the motion picture for each set time period.
If there is such a restriction, then when motion picture data is transferred via a telephone line as in the past, acceptable motion picture data cannot be obtained on the side receiving the motion picture data.
Therefore, optical fibers have been recently used as a transfer medium which can transfer large volumes of data per hour. When an optical fiber is used, it is necessary to check the line condition at the time of laying the optical fiber and to maintain it regularly.
As a apparatus for this maintenance, there have been developed various apparatuses for the detection of problems in the optical fiber, based on the backscattering light produced upon incidence of an optical pulse with the optical fiber. These apparatuses can be used also in multi-branched optical lines in which a single optical fiber is branched by a light-branching apparatus or the like. When such apparatuses measure the multi-branched optical line, it is necessary to provide a filter which reflects a test light to each branched optical line.
Briefly explaining the outline of such a apparatus, an optical pulse serving as a test light is projected into an optical fiber to be measured. The backscattering light produced in the optical fiber is then detected in time series, and this operation is regularly repeated to obtain a measurement result for one time. This measurement result is then stored, and the measurement result obtained by the next measurement is compared with that of the previous measurement. If there is any difference between these measurement results, it is determined that there is a problem in the optical fiber line.
The above-mentioned conventional multi-branched optical line testing apparatus can detect faults in optical lines, but cannot detect in which position the fault exists in the faulty line. Moreover, with the conventional multi-branched optical line testing apparatus, since each of the reflected lights observed using an OTDR (Optical Time-domain Reflectometer) measuring apparatus must not be overlapped on each other on a time axis, the arrangement position of each filter must be determined so that the distances from a coupler differ mutually. To that end, the optical fiber must be reasonably long. Using such a long optical fiber however, makes the cost of the transfer system very high, making it difficult to make this practicable.